(a) Compare interstitial and vacancy atomic mechanisms for diffusion. (b) Cite two reasons why interstitial diffusion is normally more rapid than vacancy diffusion 2. Carbon diffuses in iron via an interstitial mechanism—for FCC iron from one octahedral site to an adjacent one. In Section 5.4 (Figure 5.6a), we note that two general sets of point coordinates for this site are 0 1 2 1 and 1 2 1 2 1 2. Specify the family of crystallographic directions in which this diffusion of carbon in FCC iron takes p
The Correct Answer and Explanation is :
1. (a) Comparison of Interstitial and Vacancy Atomic Mechanisms for Diffusion:
Interstitial Diffusion:
In interstitial diffusion, smaller atoms, such as carbon or hydrogen, move through the interstitial spaces between the host atoms (which are usually larger). These interstitial atoms don’t replace the host atoms; rather, they move from one interstitial site to another. This mechanism typically occurs faster than vacancy diffusion because the interstitial atoms encounter fewer obstacles when they diffuse.
Vacancy Diffusion:
In vacancy diffusion, an atom from the host lattice moves into a nearby vacant site (a vacancy). This process involves the displacement of both the diffusing atom and the vacancy. The atom moves to an adjacent position, and the vacancy moves to the previous location of the atom. This mechanism relies on the presence of vacancies and occurs slower compared to interstitial diffusion since it involves atomic rearrangements.
1. (b) Reasons Why Interstitial Diffusion is More Rapid Than Vacancy Diffusion:
- Size of Diffusing Atoms:
Interstitial diffusion involves smaller atoms (e.g., carbon or hydrogen) that can move through the empty spaces between host atoms. Because these smaller atoms do not need to displace larger atoms or involve any atomic vacancy rearrangements, their movement is faster. In contrast, vacancy diffusion requires the diffusion atom to jump from one site to another, requiring more energy for the larger atoms to shift and displace. - Energy Barriers:
In vacancy diffusion, the atom must overcome the energy barrier required to move from its original lattice site into the adjacent vacancy site, which is often higher than the energy barrier for interstitial atoms to jump between interstitial sites. Interstitial atoms face fewer obstacles in their movement, leading to lower activation energy and, thus, faster diffusion.
2. Carbon Diffusion in FCC Iron via Interstitial Mechanism:
In FCC iron, carbon atoms diffuse through the octahedral interstitial sites. The two crystallographic points 0.5, 0.5, 0.5 and 1, 0.5, 0.5 describe the locations of these interstitial sites. The general family of crystallographic directions along which this diffusion of carbon occurs can be determined from the symmetry and position of the interstitial sites.
The directions along which carbon atoms diffuse in FCC iron are the ⟨110⟩ directions. This is because the interstitial sites form a pattern where carbon atoms can diffuse from one octahedral site to another, typically along these close-packed directions.
Explanation:
- In an FCC crystal structure, the octahedral interstitial sites are located between the host atoms, with certain crystallographic directions enabling efficient diffusion.
- The FCC lattice is highly symmetric, and the shortest, most favorable paths for diffusion are along the ⟨110⟩ directions because they connect the nearest interstitial sites.
- As carbon atoms move through these interstitial sites, they follow the lattice’s inherent symmetry, leading them to travel most efficiently along these directions.
Thus, the family of crystallographic directions in which carbon diffuses in FCC iron is ⟨110⟩.